An exhaust-gas recirculation system for use in an automotive internal combustion engine is well known per se and is generally adapted to recirculate the exhaust gases of the power cylinders of an internal combustion engine into the mixture induction system of the engine usually at a controlled rate. An emission control system of this type is useful primarily for the reduction of toxic nitrogen oxides to be produced as a result of the combustion of an air-fuel mixture in the combustion chambers of an internal combustion engine. On the other hand, a secondary-air injection system is adapted to inject secondary air into the exhaust gases in the exhaust system of an internal combustion engine usually at a controlled rate so as to promote the re-combustion or "afterburning" of toxic combustible residues such as hydrocarbons and carbon monoxide contained in the exhaust gases discharged from the combustion chambers of the engine. An emission control system of this particular nature is also well known per se and is useful for the control of hydrocarbon and carbon monoxide emissions especially when used in combination with positive exhaust-gas reoxidizing means such as a thermal reactor provided in the exhaust system of an internal combustion engine.
With a view to reducing all of the above-mentioned types of air contaminants, viz., nitrogen oxides, hydrocarbons and carbon monoxide in the exhaust emissions of an internal combustion engine, some modernized automotive internal combustion engines use both the exhaust-gas recirculation systems and the secondary-air injection systems. In order that the exhaust-gas recirculation and secondary-air injection systems thus incorporated in combination into an internal combustion engine be enabled to fully exhibit their respective potential performance efficiencies, it is a matter of prime importance where the exhaust-gas inlet port of the former system and the secondary-air outlet port of the latter system should be located in the exhaust system of the engine. If, for example, the secondary-air outlet port is located upstream of the exhaust-gas inlet port, the exhaust gases to be recirculated into the mixture induction system of the engine contain in a certain proportion the air which has not been consumed in the process of re-combustion in the exhaust system upstream of the exhaust-gas inlet port. This causes excessive dilution of the air-fuel mixture to be produced in the mixture induction system of the engine and gives rise to an increase in the concentration of nitrogen oxides in the exhaust gases to be emitted from the combustion chambers of the engine. If, conversely, the exhaust-gas inlet port of the exhaust-gas recirculation system is located in the exhaust system upstream of the secondary-air outlet port of the secondary-air injection system, then the problem above pointed out would be solved but there is encountered a problem that the efficiency at which the exhaust gases are to be re-combusted with the aid of the secondary air in the exhaust system tends to be critically impaired. If, in this instance, the exhaust system is arranged with positive exhaust-gas re-oxidizing means such as a thermal reactor, the secondary air is admixed with the exhaust gases which have been processed in such re-oxidizing means and, as a corollary, the provision of the secondary-air injection system could not bring significant results.
To avoid these problems resulting from the above described port arrangements of the exhaust-gas recirculation and secondary-air injection systems, the exhaust-gas inlet port may be disposed to be open directly to the exhaust port of one of the power cylinders of an engine. As will be explained in more detail, however, such a port arrangement necessitates a disproportionately large-sized construction of an engine body and is not acceptable from a practical point of view.
The present invention therefore contemplates elimination of all the above described drawbacks of the ordinarily conceivable port arrangements of the exhaust-gas recirculation and secondary-air injection systems incorporated in combination into an automotive internal combustion engine.
It is, accordingly, an important object of the present invention to provide an automotive multi-cylinder internal combustion engine featuring the combination of exhaust-gas recirculation and secondary-air injection systems which are arranged to be capable of fully exhibiting their respective potential performance efficiencies.
It is another important object of the present invention to provide an automotive multi-cylinder internal combustion engine equipped with exhaust-gas recirculation and secondary-air injection systems which are arranged in such a manner that the operation of each of the systems is not affected or impaired by the operation of the other system.
Yet, it is another important object of the present invention to provide an automotive multi-cylinder internal combustion engine having emission control systems which are capable of reducing nitrogen oxide, hydrocarbon and carbon monoxide emissions of the engine to satisfactory degrees substantially throughout various modes of operation of the engine.